Tacrolimus, or FK506, is a potent immunosuppressant increasingly used for preventing rejection of transplanted organs in humans. However, the bioavailability of tacrolimus varies considerably between individuals, primarily because of variable metabolism by the cytochrome P450 3A enzymes. The goal of this project is to engineer the tacrolimus biosynthetic genes in a Streptomyces strain to obtain specific analogs resistant to the major degradative pathways and very difficult to obtain by other means. The modifications we propose will lead to proprietary analogs that should be more resistant to degradation, possibly less toxic, and fully active as immunosuppressants. The technology for engineering macrolide biosynthetic genes is well established at Kosan. Once the analogs have been produced, they will be compared with tacrolimus for susceptibility to P450 degradation, for in vitro activities predictive of immunosuppressant activity, and for stability and toxicity in rats. The proposed analogs represent novel immunosuppressants that could significantly improve quality of life for transplant recipients. PROPOSED COMMERCIAL APPLICATIONS: The market for immunosuppressants is large and growing, yet the currently available compounds have drawbacks such as poor and variable bioavailability. If these analogs prove to be superior as proposed, they could capture a significant share of the immunosuppressant market.